Traditional projection displays use an electrically powered arc lamp with reflective surfaces and focusing lenses to produce a high intensity beam of light. This light beam is imaged using a spatial light modulator and a series of imaging optical elements. Common spatial light modulators include liquid crystal based systems and electrically controllable micromirror arrays.
Some current alternative approaches generally use a package of high power LEDs as the light source. The light emitted by such a source is directed with the aide of focusing optics into a single optical waveguide, such as a large core plastic optical fiber, that transmits the light to a location that is remote from the source. In yet another approach, the single fiber may be replaced by a bundle of individual optical fibers.
Projection displays often have a much lower dynamic illumination range than is perceptible by the eye. The illumination range is usually described by the contrast ratio of the display, where the contrast ratio is the output in the brightest state compared to the output in the darkest state.
One approach for increasing the contrast ratio of projection displays is to put two spatial light modulators in series. One of these modulators may have equal or lower resolution than the other. The contrast ratio of two modulators in series is approximately equal to the product of the individual modulators.